Self-propelled agricultural harvesting machines, for example combines and forage harvesters, are usually driven by means of internal combustion engines. In the simplest case the rotational speed of the internal combustion engine is provided as an input by the operator, for the purpose of which rotary knobs or other operating elements are provided, whose position is detected by a sensor and transmitted to an electronic engine control arrangement. The electronic engine control arrangement then adjusts the rotational speed of the internal combustion engine according to the operator's input. During the harvesting process the operator adjusts to a relatively high engine rotational speed suggested by the manufacturer. The rotational speed of the internal combustion engine is then lowered during the harvesting operation as a function of the actual harvested crop throughput, where the operator endeavors to maintain an appropriate engine rotational speed that is slightly below the adjusted engine rotational speed for the forward propulsion velocity provided as input by means of an operating lever. Accordingly frequent changes in the forward propulsion velocity are necessitated by variations in the density of the stand of the harvested crop on the field. This requires the constant attention of the operator and has been shown to be tiring for the operator. The engine rotational speed can be lowered to the point of stopping the engine by very high loads of the harvesting machine occasioned by dense stands of the crop and/or excessively high forward propulsion velocity.
It has been proposed furthermore (DE 35 05 887 A1) that the rotational speed of the chopper drum and the drive torque of the preceding units be detected and to lower the forward propulsion velocity, when the rotational speed of the chopper drum is below a certain value or the drive torque is above a certain value. Analogously the forward propulsion velocity is increased when certain values of the rotational speed are exceeded or the drive torque is below certain values. Thereby the rotational speed of the combustion engine is held at all times, within a range of rotational speeds that lies below the maximum rotational speed. In this arrangement the fuel supply for the internal combustion engine and its rotational speed is not changed by the control arrangement. The rotational speed of the internal combustion engine remains relatively high even in times of relatively low load, for example, during the turn at the end of the field while stopped or during operation on public roads, which leads to excessively high fuel consumption. A similar control arrangement is seen in EP 1 419 687 A1, where the forward propulsion velocity depends upon the power developed for the main unit (for example, a threshing drum or chopper drum) and for the auxiliary units, for example, the forward propulsion drive, and is adjusted in such a way that the maximum power output of the internal combustion engine is not exceeded.
In addition EP 1 275 290 A1 proposed that the rotational speed of the internal combustion engine of a forage harvester be selected as a function of the type of the front harvesting attachment and the length of the exhaust stack of the forage harvester.
EP 1 232 682 A1 proposes that the power output curve of the engine control of an internal combustion engine of a combine be selected as a function of the type of the front harvesting attachment.
EP 1 236 389 A1 proposes that the rotational speed of the internal combustion engine of a forage harvester be controlled as variable by the operator, but also to provide a lower limit of the rotational speed in order to avoid the exhaust blower from jamming which was operated at a constant gear ratio with the internal combustion engine.
EP 0 901 928 A2 proposes that the load required by the output drive of the internal combustion engine be detected and to reduce the rotational speed of the internal combustion engine when a threshold value is exceeded.
EP 1 609 349 A1 proposes that the internal combustion engine of a self-propelled harvesting machine be operated at a variable rotational speed that is a function of the actual throughput. The forward propulsion velocity of the harvesting machine is provided as input by the operator and is independent of the throughput.
DE 10 2007 004 576 A1 describes a forage harvester in which the engine rotational speed is reduced at relatively high harvested crop throughput in comparison to the maximum rotational speed, while the forward propulsion velocity is increased in order to reduce the fuel consumption. This utilizes the fact that lower acceleration of the harvested crop is sufficient for the higher throughput values in order to expel the harvested crop from the harvesting machine.
EP 1 818 524 A1 describes a self-propelled forage harvester with two internal combustion engines. A first internal combustion engine is operated at its maximum output, while at part load operation the second internal combustion engine is operated at the same rotational speed with lower power output, so that both engines operate along the same fuel consumption characteristics.
DD 200 234 A1 proposes that the engine rotational speed of the internal combustion engine of a combine be reduced during operation on public roads relative to the rotational speed during the harvesting operation, where the operation on public roads is recognized on the basis of the associated gear ratio of a shifted gear box. Both rotational speeds are provided as a fixed input.
EP 1 052 388 A2 proposes that the rotational speed of the internal combustion engine of a self-propelled harvesting machine be adjusted during operation on public roads on the basis of the forward propulsion velocity provided as input. The operation on public roads is recognized on basis of the position of an operating element that differs during operation on public roads from that during the harvesting operation. When the harvesting machine is stopped the engine rotational speed is lower than during operation of the harvesting machine.
Accordingly it is common practice in the state of the art to maintain the rotational speed and the power output of the internal combustion engine of a self-propelled harvesting machine as constant as possible, that is performed by automatically varying the forward propulsion velocity of the harvesting machine (DE 35 05 887 A1, EP 1 419 687 A1, EP 0 901 928 A2) or to maintain the forward propulsion velocity as constant and to vary the rotational speed of the internal combustion engine (EP 1 609 349 A1). In addition, it is common practice to vary the rotational speed of the internal combustion engine in order to make the power output conform to varying harvest crop throughput values. Thereby the rotational speed of the internal combustion engine is reduced relative to the nominal rotational speed when the power output is to be reduced relative to the nominal power output (compare EP 1 232 682 A1, EP 1 236 389 A1, EP 1 275 290 A1, and DE 10 2007 004 576 A1) Particularly during the harvesting operation this results in the disadvantage that the rotational speed of the harvested crop processing element is not reduced unless they are driven by costly gear boxes at constant speeds independently of the rotational speed of the internal combustion engine, which, however, leads to less than optimum harvested crop processing and harvested crop conveying. An automatic reduction of the rotational speed as a function of a known operating mode of the harvesting machine has so far been proposed only for operation on public roads (DD 200 234 A1, EP 1 052 388 A2). It would also be useful during the harvesting operation to lower the rotational speed of the internal combustion engine relative to the rotational speed selected for full power operation if the harvesting machine is running or is stopped, for example, ahead of the turn at the head of a field in order to wait for a transport vehicle upon which the harvested crop is to be transferred.